Method for making uniform single crystal semiconductors epitaxially

ABSTRACT

A liquid epitaxial method for growing single crystal semiconductors, such as gallium arsenide, having a substantially uniform three-dimensional impurity distribution. A melt of the epitaxial growth material is first caused to etch the surface of the seed to prepare a virgin surface. Epitaxial formation then involves rapid nucleation of the semiconductor on the virgin surface so that nucleation occurs uniformly over the entire seed surface. Thereafter, formation of the bulk semiconductor proceeds isothermally by vapor-liquid-solid deposition to insure uniformity of formation of the semiconductor.

United States Patent 1 Suzuki [4 1 Jan. 30, 1973 I54] METHOD FOR MAKING UNIFORM SINGLE CRYSTAL SEMICONDUCTORS EPITAXIALLY [75] Inventor: Clarence K. Suzuki, Huntington Beach, Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: April 2, 1969 [21] Appl. No.:.812,684

[52] U.S. Cl. ..l48/l.5, 23/301 SP, 117/106, 117/201, 148/171, 148/172, 252/623 GA [51] Int. Cl. ..H0ll 7/38, B0 lj' 17/20 [58] Field of Search ..l48/1.5, 1.6, 171-173; 117/106, 201; 252/623 GA; 23/273, 301,

[56] References Cited OTHER PUBLICATIONS Nelson, H. Epitaxial Growth From the Liquid State-DiodesR.C.A. Review, V. 24, Dec. 1963, pp. 603-615 Rupprecht, H. New Aspects Of Solution Regrowth-Gallium Arsenide Proc. 1966 Symp. on GaAs, Reading, Paper No. 9, pp 57-61 Primary Examiner-Richard 0. Dean Assistant Examiner-W. G. Saba Attorney-W. H. MacAllister and Lewis B. Sternfels [57] ABSTRACT A liquid epitaxial method for growing single crystal semiconductors, such as gallium arsenide, having a substantially uniform three-dimensional impurity distribution. A melt of the epitaxial growth material is first caused to etch the surface of the seed we a virgin surface. Epitaxial formau-.. men involves rapid nucleation of the semiconductor on the virgin surface so that nucleation occurs uniformly over the entire seed surface. Thereafter, formation of the bulk semiconductor proceeds isothermally by vapor-liquidsolid deposition to insure uniformity of formation of the semiconductor.

7 Claims, 2 Drawing Figures PATENTEUJAH 30 I975 Fig. l.

Fig.2.

Time

Clarence K. Suzuki,

INVENTOR.

ATTORNEY.

A METHOD FOR MAKING UNIFORM SINGLE CRYSTAL SEMICONDUCTORS EPITAXIALLY This invention relates to epitaxial growth of single crystal semiconductors and more particularly to liquid epitaxial growth of uniform single crystal semiconductors under isothermal conditions.

Epitaxial growth of semiconductors, particularly binary intermetallics, has flourished in recent years. As more uses are found for these semiconductors, more data is accumulated on them facilitating new and better ways of forming them. However, not all methodsof forming these semiconductors are useful for all the various applications of the end product. Some applications require the semiconductor to be highly doped with impurities such as donors and acceptors, others not highly doped, and others not doped to any significant extent at all, this last type being commonly referred to as intrinsic material. Certain applications of the material require it to have a fast electrical response as, for example, in switching devices, while other applications require the materials to have certain energy band configurations to facilitate light emission as in the case of lasers. Whereas any one method for forming these semiconductors may produce ideal characteristics for a particular application of the semiconductor, it may also produce undesirable characteristics for other applications. Indeed the very same characteristics which are desirable in the one case may well be undesirable in another.

As a result of this situation, new processes for making semiconductors are being developed and improved constantly. As an example, the process of epitaxial growth has been used widely in forming semiconductors.

A conventional epitaxial process for forming a semiconductor, such as is exemplified in the synthesization of gallium'arsenide, consists of first saturating a high temperature solution of gallium with crystals of gallium arsenide. This saturation can be accomplished by either introducing existing crystals into the solution or introducing arsenic vapor above the solution thereby causing the gallium to react with the arsenic vapor to form the gallium arsenide in the solution. In either case, the solution is saturated. At this point, a seed of single crystal gallium arsenide is introduced into the solution. The temperature ofthe solution is lowered so that nucleation of the semiconductor on the seed can occur. The growth process is allowed to continue for as long as is necessary to form the desired size single crystal semiconductor.

Semiconductors formed by conventional epitaxial methods can be used to advantage in devices such as diodes and semiconductors. 'P-N junctions can be formed during conventional epitaxial growth by controlling the impurities introduced into the epitaxial solution.

There are certain situations where semiconductors are used, such as in microwave applications wherein certain of the semiconductor parameters are critical for optimum performanceof the overall system. For example, for maximum efficiency of operation of Gunn oscillators or Limited Space-Charge Accumulation oscillators, the resistance, resistivity, mobility and carrier concentration of the'semiconductor element used to generate the necessary frequency signals should all be substantially uniform throughout the semiconductor element. To achieve this uniformity, there must be three-dimensional uniformity of impurity atoms within the semiconductor.

Conventional epitaxial methods of forming semiconductors do not provide such uniformity because of the non-uniform nucleation on the seed crystal, which results in three-dimensional non-uniformity of impurity atoms within the semiconductor.

Therefore, it is an object of the present invention to provide an improved method for making semiconductors.

It is another object of the present invention to provide a method for forming semiconductors having a substantially three-dimensional uniformity of distribution of impurity atoms.

A further object of the present invention is to provide an epitaxial method for making semiconductors wherein the nucleation takes place with substantial uniformity over the seed.

Yet another object of the present invention is to provide a method for forming semiconductors having substantially uniform .resistance, resistivity, mobility and carrier concentration.

These and other objects and advantages of the present invention are provided by a process for making single crystal semiconductor material such as, but not limited to, gallium arsenide wherein a'solution of gallium is saturated with gallium arsenide crystals in the conventional manner at a saturation temperature higher than the minimum temperature of the gallium arsenide. An alternative method of furnishing gallium arsenide into the solution is by introducing arsenic which then combines with some of the gallium in the solution. A seed of single crystal gallium arsenide is then covered by this solution, and its temperature is raised above the temperature of the solution so as to melt part of the seed, thereby exposing a virgin surface of the seed. At this point, the seed temperature is lowered rapidly to initiate rapid nucleation. The rapidity here insures uniformity of nucleation on the virgin surface of the seed. The seed temperature is then stabilized to allow isothermal growth for as long as is needed to achieve the desired crystal length.

The invention will be described in greater detail by reference to the drawings in which:

FIG. 1 is a cross sectional elevationalview of the apparatus used in the invention; and

FIG. 2 is a temperature versus time representation of the process of the invention.

- Referring now to FIG. I, there is shown a quartz tube 10 partially enclosed in a furnace 12. A top heating element 14 and a bottom heating element 16 are furnished in order to heat the upper and lower portions respectively, of the tube 10. A quartz platform 18 is shown positioned in the middle of tube 10 with a graphite boat 20 sitting thereon. For best results, the boat 20 is positioned in the upper half of the tube 10 so as to be nearest the top heating element 14. Within the boat 20 is a recess 22 wherein is contained a seed 24 of single crystal gallium arsenide. A seed cover 26 having a concave edge 27 is positioned in the boat 20 so as to cover the recess 22. A pool 28 of gallium and crystals 29 of gallium arsenide are disposed in the boat 20 near the recess 22 and a container 30 containing arsenic is situated in the tube near the platform 18. Hydrogen gas 32 is shown flowing through the tube 10.

The various steps involved in the invention can best be described by reference to FIG. 2 in conjunction with FIG. 1. FIG. 2 shows a first plot 100 of the temperature of the portion of the quartz tube 10 nearest the lower heating element 16 and a second plot 200 of the temperature of the portion of the tube 10 nearest the upper heating element 14 as functions. of time. These temperatures are measured by thermocouples (not shown) and indicate approximately the temperatures of the seed 24 and the upper part of the gallium pool 28, respectively. A 10 gram supply of gallium is placed in the graphite boat 20 at room temperature, Crystals of gallium arsenide 29 are added to the gallium 28. The single. crystal seed 24 of gallium arsenide is placed in the recess 22 of the boat 20 and is covered by the seed cover 26. The boat 20 is then placed on the platform 18 in the quartz tube 10. Along side the platform 18 is placed a source of arsenic 30. At time t =0, power to the top and bottom heating elements 14 and 16 is turned on, the voltage sources (not shown) being set at 130 volts. Since the distance between the gallium pool 28 and the top heating element 14 is shorter than the distance between the seed 24 and either heating element, the seed 24 will be at a lower temperature than the gallium pool 28. The top of the gallium pool 28, however, will be colder than thebottom because of exposure to the ambient hydrogen gas stream 32.

When the temperature inside the tube goes above 30 C, the gallium forms a liquid pool 28 as shown. When the temperature of the gallium pool 28 reaches 600 C, the over-pressure of the hydrogen 32 being atmosphere, the crystals 29 start to dissolve in the gallium, forming a solution. Once the gallium temperature is high enough to dissolve all the crystals 29 (-900 C), it is reduced to approximately850 C so that a small portion of the dissolved crystals 29 precipitates back out of, the solution. This ensures that the solution will be 100 percent saturated with gallium arsenide. The importance of having the solution 100 percent saturated will become clear subsequently. 1

Region I in FIG. 2 depicts the rise in temperature of the solution to approximately 900 C, the point where all the crystals 29 are completely dissolved in the gallium. Region II depicts the aforementioned reduction in the temperature of the seed 24. At this point, approximately 850 C, the system is allowed to come to equilibrium for about an hour.

Region III depicts a reduction in the power to the lower heating element 16, causing a decrease in temperature of theseed 24 to about 830? C, followed by a period of stabilization of approximately 2 hours wherein the system is again allowed to come to equilibrium. Here, the furnace 12 is tilted causing the solution to flow against the concave edge 27 of the seed cover 26 in turn causing the seed cover 26 to slideaway from the recess 22 and allowing the gallium solution to pour into the recess 22 and onto the seed 24. The edge 27 of the seed cover 26 is curved in order to prevent the solution from flowing over the seed cover 26. If the solution were to flow over, the seed cover 26 might remain over the recess, preventing the solution from coming in contact with the seed 24. At this point, some of the gallium arsenide in the solution will precipitate out on the seed as a result of the combination of two occurences: First, the solution has been cooled slightly by virtue of exposure to the ambient hydrogen gas flow when the solution flowed into the recess 22 and, second, the seed temperature had been lowered prior to tilting the furnace, thereby further cooling the solution as it contacts the seed 24. As a result of the cooling of the solution in the recess, the solution becomes supersaturated to permit precipitation of the gallium arsenide out of the solution. These two occurrences or conditions are important for, had the solution not been percent saturated, it might have partially dissolved the seed 22 into the solution causing premature etching of the seed and resulting in a non-planar seed surface which is undesirable for uniform impurity distribution in the single crystal growth. Furthermore, had the seed temperature not been lowered before tilting the furnace 12, the solution might not have been cooled sufficiently to cause the precipitation.

The power to the lower heating element 16 is increased by setting the voltage to about volts for 30 seconds as depicted by Region IV. This increases the seed temperature so that the surface of the seed 24 is etched in a controlled manner as desired, creating a virgin surface on which single crystal growth can occur. Thus, the gallium arsenide previously precipitated on a contaminated seed surface is now dissolved back into solution.

Region V depicts the shutting off of power to the lower heating element 16 creating a temperature gradient in the solution, the temperature being lower at the bottom, and causing a decrease in the seed temperature and the solution temperature at'a rate of approximately 20lmin in the first minute, l2/min in the second minute, 7/min in the third, and so on with A temperature/A time becomingsmaller asymptotically. Uniform nucleation of single crystal gallium arsenide on the seed 24 occurs'in this interval defined by Region V, the seed temperature at the start of this region being the nucleation temperature. The temperaturereduction rate of the seed 24 covered by the gallium solution cannot proceed, too rapidly or polycrystals would form in the solution and a single crystal growth would not be obtainedfNeither can the'temperature reduction proceed too slowly because then the gallium arsenidev would nucleate preferentially on defectsites on the seed, giving rise toa non-planar single crystal having a non-uniform impurity distribution.

The rate of. temperature reduction was chosen so that it would be rapid enough to cause nucleation on the seed surface uniformly, i.e., so that the gallium arsenide in the solution would not 'detect and be preferentially attracted to the defect sites on the seed surface. The seed temperature reduction rate was chosen to compliment the saturation temperature. Had the saturation temperature been l,200 C instead of 900 C, the seed temperature reduction rate would have been smaller, i.e., approximately 5/min for the first minute, whereas it would have been much larger,i.e., --45/min had the saturation temperature been -700 C. The invention is not limited to any particular saturation temperature so long as it is sufficiently high to allow for a rapid s'eed temperature reduction rate without the solution temperature falling below the point where it can dissolve the solute.

The critical solution temperature is defined as that temperature below which the solute will not dissolve in the solvent. The lowest temperature above the critical solution temperature that will allow a sufficiently rapid seed temperature reduction rate for uniform nucleation is defined herein and in the claims as the minimum nucleation temperature. For gallium arsenide, the minimum nucleation temperature is approximately 675 C, therefore thegallium solution saturation temperature should be greater than 700 C. If the gallium solution saturation temperature were below 675 C, the initial rapid temperature reduction rate would bring the temperature below 600 C in about 2 or 3 minutes, which means that, after the initial nucleation occurs, the solvent can no longer contain the dissolved solute, thus precluding further single crystal growth.

The reason for a 100 percent saturated solution can thus be fully appreciated. Since the seed is always cooler than the solution, that part of the solution in contact with the seed is cooler than the rest of the solution. The amount of solute that can be dissolved in a solvent is dependent on temperature. The lower the solvent temperature, the less solute it can hold. Thus, as the seed temperature falls, that part of the solution in contact with it can hold less gallium arsenide than the rest of the solution and, therefore, the gallium arsenide precipitates out of solution on the seed 22 in single crystal form instead of elsewhere in the solution in polycrystalline form. 7

Region VI represents the growth of the single crystal gallium arsenide under substantially isothermal conditions. Once the initial uniform nucleation occurs, continued uniform growth will follow if the process continues isothermally thereafter. Variations in the temperature of the growth surface could alter the growth patterns such that growth is non-uniform.

So long as the gallium solution is fully saturated, epitaxial single crystal growth will occur. In order to maintain 100 percent saturation, the source of arsenic 30 is furnished. The high temperature in the tube 10 causes the arsenic to vaporize. A flow of hydrogen through the tube 10 is constantly maintained throughout the process in order to transport the arsenic vapor to the gallium solution. The arsenic reacts with the gallium at the top surface of the solution to form gallium arsenide at a rate sufficient to keep the solution 100 percent saturated. Thus, the growth can continue until the gallium solution is exhausted, assuming a sufficient supply of arsenic. lf gallium arsenide crystals are not available, the arsenic source can be used to form the solute in the solution initially.

Region VII represents the end of the growth.'The powerto the heating elements 14 and 16 are shut off and any gallium arsenide grown during this cool-down period is discarded, as is the amount of gallium arsenide grown during the initial nucleation period. Only the bulk epitaxial material grown in between nucleation and furnace shut-down would possess uniform three-dimensional dopant distribution.

Although the inventive process has been described specifically for growing gallium arsenide, it is equally applicable for growth of other semiconductor compounds from the lIl-V group such as indium antimonide, gallium phosphide and indium phosphide, semiconductor compounds of the Il-Vl group and IV- VI group such as zinc selenide, lead telluride or tin telluride, and P-type silicon when aluminum is the solvent. These and other combinations are capable of such epitaxial growth so long as the two materials comprising the solute and solvent do not react chemically with one another and so long as the solute vcan be dissolved in the metal solvent to form a solution. in addition, the process is not limited to growth of a semiconductor comprising two elements. For example, gallium arsenide phosphide growth is obtainable if crystals of gallium arsenide and gallium phosphide are dissolved in gallium. In all the foregoing examples, the temperatures involved must be made to match the characteristics of the materials used.

The-invention is also workable with a eutectic composition of two or more metals. For example, germanium can be grown from a gold-germanium eutectic at temperatures above 356 C.

There has thus been shown and described a method for epitaxially growing single crystal semiconductor material having a substantially uniform three-dimensional impurity distribution.

Although specific embodiments of the invention have been described in detail, other variations of the embodiments shown may be made within the spirit and scope of the invention.

Accordingly, it is intended that the foregoingdisclosure and drawings shall be considered only as illustrations of the principles of this invention and are not to be construed in a limiting sense.

What is claimed is:

l. A process for growing singlev crystal semiconductor gallium arsenide in an open tube and in an atmosphere of a gas which is non-reactive with gallium arsenide, comprising the steps of:

a. forming a saturated solution of gallium and a charge of gallium arsenide at a saturation temperature higher than the minimum nucleation temperature of gallium arsenide;

b. covering a seed of gallium arsenide with the saturated solution while maintaining the seed at a temperature lower than of the solution;

c. increasing the temperature of the seed and the saturated solution such that the saturated solution is converted into a nonsaturated solution, thereby allowing a surface part of the seed to be dissolved in the nonsaturated solution and creating a virgin surface on the seed for subsequent nucleation;

d. lowering the temperature of the seed below the temperature of the top of the nonsaturated solution to create a temperature gradient from the top of the nonsaturated solution to the seed, lowering the temperature of the seed and the nonsaturated solution substantially equally at a predetermined rate to convert the nonsaturated solution into a saturated solution, and adding arsenic to the saturated solution from a source of arsenic for maintaining saturation of the saturated solution during the subsequent nucleation, the gradient and the equal rate of decline of the seed temperature and the solution temperature causing substantially uniform nucleation of gallium arsenide on the virgin surface of the seed; and

e. stabilizing the seed temperature and the solution temperature and maintaining the gradient and the stabilized temperatures during formation of the single crystal while maintaining the saturation of the saturated solution.

2. The process claimed in claim 1 wherein said. step of forming the saturated solution further comprises the step of dissolving crystalsof the gallium arsenide in the gallium solvent, the solvent having a lower melting point than the gallium arsenide.

3. The process claimed in claim 1 wherein said step of forming the saturated solution further comprises the steps of liquefying the gallium solvent and then causing vaporized arsenic from the arsenic source to react with part of the liquefied gallium solvent to form gallium arsenide, the remainder of the gallium solvent dissolving the gallium arsenide until the saturated solution results.v

4. A process for growing single crystal gallium arsenide in an open tube and in an atmosphere of a gas which is non-reactive with gallium arsenide, comprising the steps of:

A. removing asurface layer from a seedof gallium arsenide to provide a virgin surfaceron the seed, 'said removing step comprising the steps of:

1. covering the seed with a saturated solution of gallium and gallium arsenide, the solution being at a first temperature higher than the minimum nucleation temperature of gallium arsenide, while maintaining the seed'at a second temperature lower than the first temperature, and

2. increasing thereafter thetemperature of the seed and the saturated solution to a third temperature to convert the saturated solution into a nonsaturatedsolution to permit the surface layer to dissolve into the nonsaturated solution;

B. depositing gallium arsenide on the virgin surface by lowering the temperature of the seed below the minimum nucleation temperature of gallium arsenide to commence deposition of gallium arsenide on the seed while establishing a decreasing temperature gradient of the solution from the third temperature to the minimum nucleation temperature at the seed;

C. stabilizing the temperature of the seed and the saturated solution at an isothermal temperatureto continue the deposition of gallium arsenide to uniform growth; and

D. providing sources of gallium arsenide and arsenic for replenishment of the solution during said temperature lowering and stabilization steps.

5. The process claimed in-claim 4 wherein the third temperature is at least 675C and wherein the temperature gradient is at most 10 centigrade degrees.

6.. The process claimed in claim 4 wherein the third temperature is between 700 and l,240 C and wherein the temperature gradient is between 1 and 5 Centigrade degrees.

7. The processclaimed in claim 6 wherein the solution temperature is between 800 and 950 C. 

1. covering the seed with a saturated solution of gallium and gallium arsenide, the solution being at a first temperature higher than the minimum nucleation temperature of gallium arsenide, while maintaining the seed at a second temperature lower than the first temperature, and
 1. A process for growing single crystal semiconductor gallium arsenide in an open tube and in an atmosphere of a gas which is non-reactive with gallium arsenide, comprising the steps of: a. forming a saturated solution of gallium and a charge of gallium arsenide at a saturation temperature higher than the minimum nucleation temperature of gallium arsenide; b. covering a seed of gallium arsenide with the saturated solution while maintaining the seed at a temperature lower than of the solution; c. increasing the temperature of the seed and the saturated solution such that the saturated solution is converted into a nonsaturated solution, thereby allowing a surface part of the seed to be dissolved in the nonsaturated solution and creating a virgin surface on the seed for subsequent nucleation; d. lowering the temperature of the seed below the temperature of the top of the nonsaturated solution to create a temperature gradient from the top of the nonsaturated solution to the seed, lowering the temperature of the seed and the nonsaturated solution substantially equally at a predetermined rate to convert the nonsaturated solution into a saturated solution, and adding arsenic to the saturated solution from a source of arsenic for maintaining saturation of the saturated solution during the subsequent nucleation, the gradient and the equal rate of decline of the seed temperature and the solution temperature causing substantially uniform nucleation of gallium arsenide on the virgin surface of the seed; and e. stabilizing the seed temperature and the solution temperature and maintaining the gradient and the stabilized temperatures during formation of the single crystal while maintaining the saturation of the saturated solution.
 2. The process claimed in claim 1 wherein said step of forming the saturated solution further comprises the step of dissolving crystals of the gallium arsenide in the gallium solvent, the solvent having a lower melting point than the gallium arsenide.
 2. increasing thereafter the temperature of the seed and the saturated solution to a third temperature to convert the saturated solution into a nonsaturated solution to permit the surface layer to dissolve into the nonsaturated solution; B. depositing gallium arsenide on the virgin surface by lowering the temperature of the seed below the minimum nucleation temperature of gallium arsenide to commence deposition of gallium arsenide on the seed while establishing a decreasing temperature gradient of the solution from the third temperature to the minimum nucleation temperature at the seed; C. stabilizing the temperature of the seed and the saturated solution at an isothermal temperature to continue the deposition of gallium arsenide to uniform growth; and D. providing sources of gallium arsenide and arsenic for replenishment of the solution during said temperature lowering and stabilization steps.
 3. The process claimed in claim 1 wherein said step of forming the saturated solution further comprises the steps of liquefying the gallium solvent and then causing vaporized arsenic from the arsenic source to react with part of the liquefied gallium solvent to form gallium arsenide, the remainder of the gallium solvent dissolving the gallium arsenide until the saturated solution results.
 4. A process for growing single crystal gallium arsenide in an open tube and in an atmosphere of a gas which is non-reactive with gallium arsenide, comprising the steps of: A. removing a surface layer from a seed of gallium arsenide to provide a virgin surface on the seed, said removing step comprising the steps of:
 5. The process claimed in claim 4 wherein the third temperature is at least 675* C and wherein the temperature gradient is at most 10 centigrade degrees.
 6. The process claimed in claim 4 wherein the third temperature is between 700* and 1,240* C and wherein the temperature gradient is between 1 and 5 centigrade degrees. 